How do you cast a List of supertypes to a List of subtypes?

Question

For example, lets say you have two classes:

public class TestA {}
public class TestB extends TestA{}

I have a method that returns a L

Answer 1

if you have an object of the class TestA, you can't cast it to TestB. every TestB is a TestA, but not the other way.

in the following code:

TestA a = new TestA();
TestB b = (TestB) a;

the second line would throw a ClassCastException.

you can only cast a TestA reference if the object itself is TestB. for example:

TestA a = new TestB();
TestB b = (TestB) a;

so, you may not always cast a list of TestA to a list of TestB.